There are numerous environments having need for intermixing two or more fluids in precisely controlled ratios including some where it is essential to provide for varying the proportions over a small if not a large range including commercial, industrial, construction, etc. illustrative of the latter environments is fire fighting equipment making use of one of the numerous fire fighting chemicals. Typically, a relatively small quantity of the chemical is required and this must be introduced from a non-pressurized container into a flowing stream of water customarily at high pressure under conditions positively safeguarding against backflow of chemical into the water supply source, particularly where the water is derived from a municipal water distribution system. Apparatus previously proposed for this and related purposes is disclosed in the following U.S. Pat. Nos.: Eads, 548,780; Sargeant, 2,543,941; Hachmeister, 2,558,681; Granberg, 2,567,997; Goodman, 3,549,048; Langstroth, 3,997,080; 3,073,256. The proportions disclosed in each of these prior proposals are subject to numerous disadvantages and shortcomings. Typically, the proposals utilize the main line water pressure to power a pump for raising the chemical agent to a suitable pressure for introduction into the main water stream. Additionally, these prior constructions lack adequate and satisfactory means for proportioning the two fluids. Some lack means of any character for varying the proportions of water and the chemical or other agent. Those having flow rate varying means have an extremely limited range of adjustability. Another disadvantageous characteristic of prior designs is the use of energy derived from the flow of one of the fluids to drive a pump operable to introduce one fluid into the first stream. This mode of driving the pump has the obvious disadvantage of impeding the flow of the first fluid and necessitates the use of a costly fluid motor in the main fluid line. It has also been proposed to place the chemical in a flexible walled envelope housed within a main housing sufficiently strong to withstand the main line pressure. Pressurization of the chemical in this manner is utilized to introduce the chemical into the main line through a calibrated orifice. This expedient is obviously unreliable for lack of a pressure differential across the orifice. It is also necessary to substitute different orifices in order to vary the flow of the chemical constituent. Another proportioner utilizes an electric motor to drive a pair of positive displacement pumps connected in series. One of these pumps is used to circulate a liquid in a closed circuit including an adjustable relief valve bypassing a selected portion of the liquid back to the source and the remainder being used to operate a diaphragm pump connected in circuit with the chemical actually being introduced into a second fluid in a desired proportion. This complex and costly system is disclosed in U.S. Pat. No. 3,073,256.